Human epidermal growth factor receptor-positive (HER2+) breast cancer represents 20% of all breast cancer cases. Around 70–80% of patients with HER2+ are diagnosed in the localised stage, according to the American Joint Committee on Cancer (AJCC) seventh criteria. But the number of cases diagnosed in the advanced stages will increase with the more recent AJCC eighth edition of the staging criteria.

These new staging criteria incorporate biological factors such as oestrogen and progesterone receptor, tumour grade and HER2 status in addition to anatomical classification to improve the prognostic efficacy. GlobalData epidemiologists expect 10% of all stage Ia–IIb cases (localised) diagnosed by AJCC seventh criteria to be reclassified to stage III (advanced) based on AJCC eighth prognostic criteria in the US.

According to AJCC seventh criteria, 20% of the HER2+/HR+ breast cancer cases were diagnosed in the advanced stages (12% in stage III and 8% in stage IV) last year. Under the AJCC eighth criteria, the proportion of cases in the US in stage III will increase to 20%, whereas the localised stage will decrease from 80% to 70% (Figure 1). The proportion of cases in stage IV will remain unchanged. The overall proportion of cases diagnosed in advanced stages was around 30% of all cases last year.

Breast cancer treatment modalities depend on the stage of cancer. Staging classification based on tumour size, node and metastasis is the bedrock of cancer diagnosis, but is insufficient when biological factors also determine the prognosis. The advantage of AJCC eighth criteria compared with the AJCC seventh is the finer stratification of stages by the addition of biological factors to the anatomical classification. This will help guide whether a cancer is operable and forecast the patient’s prognosis.

The national cancer registries in different countries have not yet reported data based on AJCC eighth staging criteria. Whether the overall survival rate of this cancer will improve with new classification remains unclear and needs to be explored in future studies. It is also of paramount interest that the US national cancer registry provides the updated staging data soon, as this will help in comparing stages.